The present invention relates to a circuit for automatically controlling the gain of a tank type oscillator, in particular a voltage controlled oscillator.
A voltage controlled oscillator is simply an oscillator whose frequency is proportional to an externally applied voltage. Integrated-circuit VCOs are usually simply R-C multivibrators in which the charging current in the capacitor is varied in response to the control input. However, switching transient durations at higher frequencies themselves become a large percentage of the period of the oscillation causing center frequency drift. The duration of the switching transients depends on circuit parasitics, circuit resistances, transistor transconductance, and transistor input resistance, which are all temperature sensitive. A more temperature resistant voltage controled oscillator circuit is one which uses a tank circuit consisting of an inductor and parallel capacitor. However, there are no known circuits which satisfactorily control the tank node peak-to-peak swing of such a circuit.
Accordingly, there is provided an improved automatic gain control for a tank type oscillator, especially a voltage controlled oscillator of the tank type.